Melt butter and mix with crumbs. Press 3/4 of the mixture in a 9-inch by 11-inch pan and refrigerate. Combine rhubarb, sugar, water, cornstarch and boil on top of stove until thickened. Cool and spread over crust. Mix marshmallows in whipped topping and spoon over rhubarb. Mix pudding as directed on package Spread over marshmallow mixture. Sprinkle rest of crumbs over top. Refrigerate overnight or all day before serving. Serves 12.